Description: This key attribute allows the user to enter a regular expression to match the import AS Path Attribute of the received update message. Note: the AS Path Attribute lists the ASs in reverse traversal order. The syntax of the regular expression is a subset of the RFC 2280. The basic syntax of a regular expression is a list of AS number separated by a space which making up a AS path. The following characters has the special meaning: '.' - Matches any AS number. '^n' - Matches the first AS item (n) in the AS Path Attribute, or in order words, matches the last AS (n) traversed in the AS path. 'n$' - Matches the last AS item (n) in the AS Path Attribute, or in order words, matches the first AS (n) traversed in the AS path. '[n1 ... n2]' - Matches any AS number that appeared in the set n1 ... n2. 'n1 - n2' - Matches any AS number between the range of n1 and n2. '()' - is supported to change the order of evaluation. 'n*' - Matches 0 or more occurrences of n, where n can be a set, range or AS number. 'n+' - Matches 1 or more occurrences of n, where n can be a set, range or AS number. 'n?' - Matches 0 or 1 occurrences of n, where n can be a AS set, range or number. The following logical operators are evaluated in decending order: 'NOT' - Given a regular expression x, NOT x matches the AS number pattern that is not matched by x. 'AND' - Given two regular expressions x and y, x AND y matches the intersection of the AS path that are matched by x and those are matched by y. 'OR' - Given two regular expressions x and y, x OR y matches the union of the AS path that are matched by x and those are matched by y. For example: The AS path '5 4 3 2 1', where 1 is the origin AS, would be matched by any one of the following regular expressions: 1$, ^5, 4 3, 5 3 - 4*, 5 [4 3 2] not10 2 1, not (6 7 8 9 10) and 5 4 .* 1 Empty regular expression implies a match on all AS path.